Q-Switching is a frequently used method of providing high intensity pulses from lasers and is conventionally achieved by the use of either electro-optic (EO) or acousto-optic (AO) devices placed inside the laser cavity. Regarding AO Q-switches, sound waves emanating from a biased transducer pair bonded to a crystal travel in a direction which is primarily perpendicular to the laser or other light beam incident on the crystal. This arrangement diffracts the beam passing through a crystal, using travelling acoustic waves. It is well known that the angle of the light beam relative to the sonic wavefront must be at or near the Bragg angle, which is generally a small angle such as less than a few degrees, specifically within the so-called acceptance angle which is centered on the Bragg angle, to obtain such diffraction. The acceptance angle is generally defined as the range of incidence angles for which the diffraction efficiency is greater than 50%.
Whether AO or EO-based, Q-switch laser devices are switched OFF (low optical transmission, high loss state) for a period in which the population inversion of the gain medium is pumped to a high value and then rapidly turned ON (high transmission, low loss state). During the OFF phase, the laser is operating below the laser threshold as the cavity loss is too great. During the ON phase, the cavity loss is suddenly reduced to a low value allowing laser operation. A large output optical pulse then results as the stored energy in the laser gain medium is released.
Improvements in laser technology in recent years have meant that the 80-85% loss modulation provided by current Q-Switch devices is sometimes insufficient to hold off modern lasers, thus being insufficient to completely prevent them from oscillating. One solution well known within the laser and acousto-optics industry to achieve higher loss modulation in laser cavities utilizes two or more spaced apart Q-Switches optically coupled in series. The second Q-Switch is used to further reduce the zero-order (undiffracted) intensity residual transmitted by the first Q-Switch.
The use of multiple spaced apart AO Q-Switches does not always provide the increase in loss modulation that might be expected. This is because the modulator Debye-Sears ratio and acceptance/separation angles have an effect on the overall efficiency of the system, and this is not always realised. When the Debye-Sears ratio (Q) is large and the Q-Switches operate in the Bragg regime, there are two possible device orientations.
As shown in FIG. 1, a first orientation conventional double Q-Switch device 100 is shown wherein the acoustic wave fronts 111 and 121 from each switch are aligned parallel to one another. The diffracted ray 125 emerging from the first Q-switch device 110 is then presented to the second Q-switch device 120 precisely at the Bragg angle. If the diffracted ray 125 falls on the active aperture of the second device 120, efficient re-diffraction of the first order beam back towards the zero order 135 will occur. In this case, it is possible for Q-Switches 110 and 120 coupled in series to be less efficient than a single Q-Switch, such as Q-switch 110 alone.
FIG. 2 shows a second orientation for a double Q-switch device 200, comprising Q-switch devices 210 and 220. The respective Q-Switches 210 and 220 are aligned such that the zero order beams enter each device at the Bragg angle (θB), but the devices 210 and 220 are oriented such that the respective acoustic waves 211 and 221 are not parallel to one another. In this case the diffracted beam 225 from the first device 210 falls on the second device 220 at a non-Bragg angle, such as an angle of 3θB. If this diffracted beam 225 is within the acceptance angle of the second device (which as noted above is centered on the Bragg angle), the first order beam 225 can be re-diffracted towards the zero order (not shown) and the overall efficiency of the device 200 reduced. This can be a problem with multiple low frequency spaced apart Q-Switches arranged in series.
Relative orientation of the respective Q-switches comprising device 100 or 200 is thus crucial since any rediffraction of the first order rays back into the zeroth order by the second Q-Switch will significantly reduce the loss modulation. Rediffraction is not the only drawback associated with using two separate Q-switches. The relative phase of the acoustic modulation must also be considered if timing jitter is to be avoided. The increase in cavity length associated with fitting two Q-switches results in an increase in pulsewidth. Moreover, two spaced apart Q switches will place four optical faces into the laser cavity leading to increased insertion loss and multiple reflections. Two Q switches also require extra plumbing for the water cooling system and RF drivers. Finally, each Q-switch must be carefully aligned at the Bragg angle while simultaneously avoiding rediffraction losses, which is often not possible.
What is needed is a robust Q-Switch design which provides short pulse widths and a loss modulation greater than the 80-85% loss modulation provided by current Q-Switch devices so that the loss produced is sufficient to hold off modern high cavity gain lasers, thus being sufficient to prevent oscillation.